1. Field of the Invention
The present invention relates to an active matrix display device and a video-signal processing device. More specifically, the present invention relates to an active matrix display device using a partial writing method or differential writing method, in which video data is written into pixels to be changed in each frame in order to display a moving picture. Also, the present invention relates to a video-signal processing device for generating/processing a video signal for realizing partial writing.
2. Description of the Related Art
Active matrix display devices, which are flat, are being developed as next-generation displays replacing CRTs. FIG. 1 is a schematic block diagram showing a general configuration of an active matrix display device of a related art. As shown in FIG. 1, the display device includes a pixel array unit 1 and a peripheral circuit unit 23 for driving the pixel array unit 1. The pixel array unit 1 and the peripheral circuit unit 23 may be formed on the same substrate, or may be formed separately. The pixel array unit 1 includes rows of gate lines X, columns of signal lines Y, and pixels provided at the intersections thereof, the pixels being arranged in a matrix pattern. Each pixel is driven by a switching element such as a TFT. The gate electrode of each TFT is connected to a corresponding gate line X, the source electrode thereof is connected to a corresponding signal line Y, and the drain electrode thereof is connected to a corresponding pixel.
The peripheral circuit unit 23 includes a vertical shift register 2X, a horizontal shift register 3Y, and a sampling switch group 31. The vertical shift register 2X sequentially selects pixels in units of rows through each gate line X. The sampling switch group 31 includes a plurality of sampling switches provided between a video line VL and the signal lines Y. A video signal is supplied to the video line VL from an external signal source. The video signal includes dot data corresponding to each pixel and has a time-series one-dimensional structure. The horizontal shift register 3Y sequentially opens/closes the sampling switches so as to sample the video signal from the video line VL to each signal line Y. Accordingly, corresponding dot data is written into pixels of a selected line on a dot-sequential basis.
As described above, in the active matrix display device of the related art, dot-sequential driving method, in which time-series one-dimensional video signal is written into pixels on a dot-sequential basis, is generally used. In some cases, line-sequential driving method may be used, in which a latch circuit of one line is provided between the sampling switch group 31 and the signal lines Y, and a video signal is written into pixels in selected rows on a line-sequential basis. In the active matrix display device of the related art, a time-series one-dimensional video input method is used as in CRTs. In this method, all pixels are dot-sequentially updated in each frame. Accordingly, a sampling clock frequency increases as the number of pixels increases.
The active matrix display device has a so-called hold characteristic, in which the luminance of pixels is maintained to the next frame. The hold characteristic causes blur in moving pictures. However, a method of using this characteristic positively and updating only interframe difference so as to display moving pictures has been proposed. This method is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2000-284755. Hereinafter, the principle of a partial rewriting method, in which only interframe difference is updated, will be briefly described with reference to FIGS. 2A to 2C. FIGS. 2A to 2C illustrate display patterns in which frames are changed from n-th frame (nF) to n+1-th frame (n+1F). In the display, a circular object is a moving object and a rectangular object is a stationary object. In FIG. 2A, a camera is fixed and the position of the moving object is changed from nF to n+1F. In this way, when a moving object exists at only a part of the display, an interframe difference component is minimized with respect to the entire display. In this case, by updating only the interframe difference, a moving picture can be displayed. In FIG. 2B, the camera is moving so as to follow a moving object. In this case, the position of stationary objects in the display relatively changes in frames. In FIG. 2C, the camera is moving independently of the motion of a moving object. In this case, the position of both of the moving object and stationary objects changes in frames. In FIGS. 2B and 2C, since the camera moves, an interframe difference component occupies the entire display in principle. However, a spatial redundancy actually exists in the display, and the ratio of difference component is reduced accordingly. Therefore, in any pattern of FIGS. 2A to 2C, a frame can be rewritten by changing only interframe difference, so that entire rewrite can be performed in a cycle of several frames to several tens of frames. By combining partial rewrite and entire rewrite, each frame can be rewritten by simply changing differential pixels. If the number of differential pixels is 10% with respect to all the pixels, a sampling clock frequency (dot clock) can be reduced to 1/10.
FIG. 3 is a block diagram showing an example of an active matrix display device in which partial writing can be performed. In FIG. 3, parts corresponding to those of the preceding example shown in FIG. 1 are denoted by the same reference numerals for clear understanding. In the display device shown in FIG. 3, a horizontal addressing circuit 3A is adopted instead of the horizontal shift register 3Y, so as to perform partial writing by a dot-sequential driving method. In the dot-sequential driving method in FIG. 1, the horizontal shift register 3Y sequentially controls open/close of the sampling switches. On the other hand, the horizontal addressing circuit 3A in FIG. 3 opens/closes only a necessary sampling switch, so that random scanning is performed. An address signal as well as a video signal is supplied to the horizontal addressing circuit 3A. The address signal specifies the position of a pixel to be rewritten. The horizontal addressing circuit 3A randomly accesses a sampling switch based on the address signal, so that dot data is written into a corresponding pixel by random access. By performing partial rewrite shown in FIG. 3, transfer rate of dot data can be advantageously increased. If the video format is 60 frames/second, 720×480 pixels, and dot-sequential scanning is adopted, then the transfer rate of dot data (dot clock) is about 25 MHz. When time-series one-dimensional input is performed as in CRT, a shift register operating in the vertical direction at about 30 KHz is required in the active matrix display device. Also, in the horizontal direction, a horizontal shift register operating at about 25 MHz is required. This is the same for still pictures and moving pictures. Herein, if the ratio of differential pixels is 10%, dot clock can be reduced to 1/10. By using this method, data transfer rate can be effectively increased, and efficient display can be performed by combining with a coding signal in a video input side.
However, since differential video is displayed, when random addressing as a memory is adopted, both of address and video must be input to a panel. Accordingly, the number of external input terminals for the display device increases. Also, in the display device side, an address decoder or the like must be provided in the horizontal addressing circuit, and thus the peripheral circuit is complicated. Therefore, the size of the peripheral circuit of the display device increases disadvantageously. Further, in random addressing, the access frequency is a dot frequency (several tens of MHz) in both horizontal and vertical directions. Therefore, reliability of an addressing operation is reduced and a propagation delay and noise caused by the length of wiring in the panel become significant. Accordingly, in a method of rewriting only interframe difference, it is not always adequate to perform random addressing to pixels to be rewritten, which should be solved.
FIG. 4 schematically shows random addressing. In FIG. 4, pixels are represented by dots. Black dots are pixels to be rewritten and white dots are pixels which need not be rewritten. The position of each pixel is defined by an absolute address, so that a pixel to be rewritten is specified by the absolute distance/direction from a reference point P. The horizontal addressing circuit 3A randomly scans the sampling switches based on the absolute address information, so as to write dot data into a desired pixel. For example, the black dots are specified by the absolute addresses: (X1, Y2), (X2, Y4), and (X3, Y6), respectively. In random addressing, however, when the next pixel is turned on, the distance and direction from the previous pixel is random. In an extreme case, for example, when scanning is performed from the upper-left to the lower-right, it is difficult for an active matrix display device having some physical areas to perform the scanning at a rate of several MHz of a dot clock, although it may be realized by a memory with a high integration.